Ok so here is my example (ish) ;)
Level x health and level x regen before unlocking the more impressive skills such as speed and adrenaline, or even have these configurable via a config.
"primary" (
skill 1 "x";
skill 2 "x";
skill 3 "x";
ect "x";
}
"secondary" { // Only after all of primary x level is complete.
skill 4 "x";
skill 5 "x";
skill 6 "x";
ect "x";
}
and maybe a third but not sure how much work this would involve.